More than 5.6 million Americans are either in prison or have served time there - and that number will continue to rise, the report shows.

By the end of 2001 one in every 37 Americans had some experience of prison, compared with one in 53 in 1974. Continuing at that rate, the proportion will increase to one in every 15 of those born in 2001.

In 2001 a sixth of African-American men were current or former prisoners, compared with one in 13 Latinos and one in 38 whites. The incarceration of women remains lower than of men but has increased at twice the rate since 1980 and shows similar racial disparities.

"Prison had become the social policy of choice for low income people of colour," says Marc Mauer, assistant director of the Sentencing Project, a group which promotes reduced reliance on imprisonment. "Nobody's stated it that way but we have inner-city areas starved of investment but no shortage of funds to build and fill prisons."

Those incarcerated for the first time accounted for two-thirds of the growth in prison population between 1974 and 2001. This is largely the result of the war on drugs and mandatory minimum sentencing: one in four inmates in federal and state prisons is in for drug-related offences, most non-violent. "Every dollar spent on drug treatment is better employed reducing crime than one spent building prisons," said Mr Mauer.

The effect of high imprisonment rates goes beyond crime to employment and enfranchisement. More than 4 million prisoners or former prisoners are denied the right to vote, and in 12 states that ban remains for life.

The prison system also ill prepares people for release, making recidivism more likely. Only about 13% of prisoners take part in a pre-release programme.

"Our contemporary prisons basically replicate the social order that produced the offenders to begin with," Mark Kleiman, a professor of public policy at the University of California at Los Angeles, told the Atlantic Monthly.